View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Elsevier - Publisher Connector Topdogy Vol. I I, pp. 205-121. Pcrgamon Press. 1971. Prio:ed in Great Britaio APPLICATIONS OF POINCARG DUALITY SPACES TO THE TOPOLOGY OF MANIFOLDS NORMAN LEVITT (Received 14 August 1970; revised 22 December 1970) THE INTENT of this paper is to apply some results in the theory of PoincarC duality spaces to the study of some questions in the topology of manifolds. The main sources of informa- tion about Poincarl duality spaces are Spivak’s paper [S] and some papers of the author [3], [4], [6]. In particular, we utilize some results on surgery and transversality in the Poincare duality category. In conjunction with a result of Peterson and Toda [7], we are able to develop the primary result to be used in the applications: THEOREM. For C-W con~plexes, integral homology may be represented by nzaps of orient& Poincare’ duality spaces. In other words, integral homology is representable in the sense of Steenrod, if one is willing to use oriented Poincart duality spaces rather than oriented manifolds. We show this in $2 below. Tn $3, we look at the cohomology of manifolds M’. We show that in a large number of cases an element a E H’(M’), k > f , is induced from the cohomolo,y of some (k + q)- dimensional space, 0 I q I r - k, if and only if it is induced from the Thorn class of a spherical fibration 5 over a q-dimensional space by a map Mr -+ T(c). In $5, we study the following questions: (1) If M” is a manifold which bounds, what is the lowest integer q so that M” = 8 Wnfl and W”+’ IS of homotopy dimension q? (2) Lf Al is a manifold with boundary, what is the lowest integer q so that M” is a codimension 0 submanifold of W”, W” of homotopy dimension q. We get some partial results on both these questions. In passing, we prove an analogous result to a theorem of Brown [l], i.e. there are no low-dimensional relations on the characteristic classes of a PoincarC duality space. Throughout, we draw heavily upon the techniques, as well as the results, of [3] and [6]. Some details of proofs are not filled in, but the reader is referred to the papers where the salient facts are to be found. A word about notation. When there is no other co-efficient group specified, homology and cohomology are assumed to have integral coefficients. If M” is a manifold, 8M” denotes its boundary and int M” = M” - JM”. By manifold, we shall mean either smooth or PL 205 206 NOR.MAN LEVITT manifold, as most of our results will apply in either case. \Ve use cX. T.Y usually to denote the unreduced cone and suspension respectively. If r(s) is a Thorn space and X a complex without a base point specified, T(<) A X means r(s) A X- where X- = X u *, * disjoint. If A c X, X/A has base point {A). Iff is a map f‘: .Y -+ Y, we shall use M, to denote the mapping cylinder. $1. SURGERY ON POINCAR~ DUALITY SP.-ICES This section is devoted to a brief exposition of certain facts concerning Poincare duality spaces and pairs (hereafter abbreviated P.D. space or P.D. pair). Recall that a P.D. space of dimension n is a finite complex M” with the following property: Let M” be em- bedded in S”+‘( (k 1arge) with regular neighborhood Rn+k; then up to homotopy, the in- clusion ZR c R is a fibration with fiber the homotopy type of Sk-‘. Similarly, a P.D. pair of dimension n is a finite pair (M”, ZM”) satisfying the following: Given a relative regular neighborhood R” fk, Unik-’ of (M”, a&1”) in D”‘k; SnTk-‘, k large, and setting JR = closure (?R- U) the diagram becomes, up to homotopy, a map of (k - 1)-spherical fibrations (the vertical inclusions are to be thought of as fibrations). We leave it to the reader to carry through the definition of cobordism between two P.D. spaces (see also [S], [9], [3], [6]). Given a P.D. space M”, the stable fiberhomotopy-type of the fibration ZR s: R is an invariant of the homotopy type of M”, which we call v(M”). (v can be thought of as a stable fibration over A4 because any such regular neighborhood R is homotopy equivalent to M.) We call v(M”) the normafjbration of M”. Similarly, one can define the normal fibration v(M”, dA4”) of a P.D. pair. In this case, 8M” is an (n - I)-dimensional P.D. space and v(C?M”) = v(M”, aM”) j dkf”. If M” is, in fact, a smooth or P.L. manifold, it is easily seen that the stable normal fibration v(M”) is just the fiberhomotopy type of the stable normal bundle. The notion of embedding a complex lip to homotopy in a P.D. space was defined and discussed in 131. Recall that a mapj‘: K -+ M” is said to be homotopic to an embedding iff there are P.D. pairs (N,, dN,) and (N, , dNz), with ZN, = diV, and homotopy equivalences f,:K-+N,, h:N, u N,+Ivf so that 2.v,= aJvZ homotopy commutes. If dim K I n - 3, we also require rr,ZN, E xsIN, under inclusion. APPLICATIOSS OF POIWARf DUALITY SPACES TO THE TOPOLOGY OF MAXFOLDS 207 We state some lemmas: 1.1 LEMMA [3]. Let n 2 2k i- 2. Let f: Sk + ikl”; then f is homotopic to an embedding. In fact, with 1VI as in the definition aboce, the inclusion dlV, c N, is an (n - k - I)-spherical jibration v,-; vf @f *v(W) is a trizialjbration. 1.2 LEMMA [a].Let K be a finite k-dimensional complex and iet v be a stable spherical fibration ocer K. Then if n 2 2k t I, there is a Poirlcar& pair (M”, Z.\f”) and a homotopy equiralence h : K -+ .Lf” so that v corresponds to v(hl”, aAl”>. Moreover, if n 2 2k -I- 2, (.If”, dM”) is unique up to homotopy. The first of these lemmas is easily seen to imply that it is possible to do surgery on n P.D. spaces, at least below dimension 2 , by direct imitation of the familiar techniques iI of surgery on smooth or PL manifolds. This may be stated as follows: 1.3 LEMMA. Let M” be a P. D. space and let f: M” -+ K be a map covered by a map of fibrations ,.(.W) --, 5. Supposef, : H*(M”) + H,(K) is onto in alldimensions I k, wherek I n/2. Supposejirrther that Kisi-connected. Then there is a normal cobordism of the map f to a map fi : M, -+ Kso thatf, is k-connected. As an example of the uses to which these results may be put, consider the space BSG which classifies stable oriented spherical fibrations. Let &I” be an oriented P.D. space; then there is a map v : M” -+ BSG corresponding to an appropriate orientation of v(M”). I .4 PROPOSITION. M" is oriented cobordant (in the sense of P. D. spaces) to an oriented P. D. space M In so that the map vI : MI” --f BSG is -connected. n-l Proof. Consider B = the - skeleton of BSG (B is finite). If y is the universal [ 2 1 orientable spherical fibration, it follows by 1.2 that there is a P.D. pair (N”, 8N”), and a homotopy equivalence/r : B 2; IV so that v(Nn, JN”) corresponds to y 1B. Let N’ = N, u N, a,vI and let M’ = M u N’. Since N’ is a “double” it bounds and hence M’ is cobordant to M. Moreover, the natural map v’ : M’ + BSG is covered by a map v(M) -+y, and is onto in n-l homology in dimensions I 2 . By 1.3, it is then possible to do surgery on M’ to cobord i 1 n-l M’ to a P.D. space Mi so that the map vr : M, --f BSG is - -connected. Q.E.D. [ 2 I Now following Brown [I], define I, = H*(BSG) as the ideal of the cohomology ring consisting of all those elements which are in the kernel of every homomorphism v* : H*(BSG) + H*(M”) where M” is an oriented P.D. space and v the classifying map for v(M). n-l 1.5 COROLLARY. I, = 0 in dimensions I - [ 2 I 208 SORMAN LEVITT Proof. In fact, we assert somethin g stronger, that is. given an element r in H’(BSG), n-l iI-, and a P.D. cobordism class, we may find a representative :1f” for that class so 2 t1 - 1 that V*(Y) # 0 in Hi(.LI”). For, by 1.4, I! may be assumed to be an L-13 -connected map ; n- 1 thus V* is an injection in all dimensions I -. 2 $2. REPRESENTABILITY OF INTEGRAL HOMOLOGY Let MSG be the spectrum obtained from the Thorn complexes of the universal orientable (k - 1)-spherical fibrations yk --t BSG(k). There is a natural map of spectra Q : MSG -+ K(Z), where K(Z) is the spectrum defined by the Eilenberg-MacLane spaces K(Z, k). This map is, in effect, defined by the integral Thorn class a’,, E H,(T(y,)) regarded as maps T(yk) --) K(Z, k).